BOSTON (Reuters) - Chipotle Mexican Grill Inc <CMG.N> shares fell to a four-year low on Thursday after testing confirmed Norovirus in a customer who ate at one of the chain's Virginia restaurants and a video of rodents crawling on the floor of a Dallas shop went viral.

Shares of the burrito chain, which has been under a microscope since it was linked to a string of food safety lapses in 2015, fell 4.5 percent to close at $356.05 on Thursday.

Chipotle has worked to win back customers and rebuild the reputation of the once-high flying chain, whose stock traded well above $700 before it was connected to E. coli, Salmonella and Norovirus outbreaks in 2015.

Testing confirmed Norovirus in one customer who ate at the Virginia Chipotle restaurant that was briefly closed this week following reports of diners falling ill, said Victor Avitto, environmental health supervisor for the Loudoun County Public Health Department, which has jurisdiction over the restaurant.

Chipotle closed the restaurant on Monday and reopened it on Wednesday, following a deep cleaning. News of the outbreak came after more than 100 diners reported symptoms on iwaspoisoned.com, a crowd-sourced website.

Chipotle expected the confirmation, since the symptoms described by customers of the Sterling restaurant were consistent with Norovirus, spokesman Chris Arnold said.

Norovirus is the No. 1 cause of illness from contaminated food in the United States, resulting in an estimated 19 million to 21 million illnesses annually, according to the Centers for Disease Control and Prevention. It is highly contagious and can spread from person to person, as well as through food prepared by an infected person. Symptoms include cramping, nausea, vomiting, and diarrhea.

Elsewhere, a customer video showing three rodents in a Chipotle restaurant in Dallas' West End neighborhood earlier this week was widely shared online.

Other diners, who witnessed the lunchtime incident, told a local television station they saw rodents falling from the restaurant's ceiling.

"A few mice did get inside one restaurant from outside due to a small structural gap in the building," Chipotle's Arnold said on Thursday.

"Our management immediately removed them and the gap was quickly repaired," said Arnold, who added that the chain had contacted guests to apologize.

Chipotle will report second-quarter results on July 25.

(Reporting by Lisa Baertlein in Boston; Editing by Meredith Mazzilli and James Dalgleish)